Unearthed Arcana - Having a multitude of options is the greatest strength (and weakness) of 3.0/3.5e, but few books did it better than Unearthed Arcana. Races, bloodlines, Paragon classes, variant classes, weapon groups, backgrounds, alternative spellcasting...it's just fantastic.
Fiendish Codex I: Hordes of the Abyss - While I actually prefer the layout of its sister-tome, the FCII: Tyrants of the Nine Hells, demons are more immediately gratifying than devils. This book (along with the Demonomicon articles in Dragon magazine) really helped to bring demons to life for me. Because of it, demons and their cults started appearing more commonly in my campaigns. I particularly liked the addition of the obyriths, also.
Complete Champion - For someone who recently ran a six year Greyhawk campaign this book is a real treat. I understand why it received such harsh criticism for fans of Forgotten Realms or Eberron, but for me it couldn't have been much better. Although it doesn't fit the mold of the other "Complete" books (it really feels like it's more for a DM than a player) I've made extensive use of it already and surely will use the Affiliation building blocks to create my own church organizations.